1. Field of the Invention
This invention relates to the production of aromatic anhydrides and aromatic esters having superior color properties. More particularly this invention relates to trimellitic anhydride (TMA) and dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC); and the preparation of high purity TMA from trimellitic acid (TMLA) and the preparation of high purity DM-2,6-NDC. The invention has particular applicability when the trimellitic acid has been produced by the oxidation of a 1,2,4-aliphatic-substituted benzene with molecular oxygen in the liquid phase and in the presence of a heavy metal catalyst.
2. Background
Aromatic anhydrides and aromatic esters are useful intermediates for the production of a wide variety of materials and compositions such as resins, polymers, health related products, etc. Trimellitic anhydride, which is made from trimellitic acid, and dimethyl-2,6-naphthalenedicarboxylate, which is conveniently prepared from 2,6-naphthalenedicarboxylic acid, are two such useful intermediates.
Trimellitic acid, the 1,2,4-benzene tricarboxylic acid, is useful as an intermediate in the production of quality plasticizers and polyester resins. For these applications, in which trimellitic acid is esterified with a monohydric or a polyhydric alcohol, the evolution of water as an esterification by-product together with the attendant difficulty of eliminating water from esterification reaction mixtures favors the desirability of employing trimellitic acid as the anhydride rather than as the acid. Resins and plasticizers may further require a trimellitic anhydride which is relatively free from color bodies and also free from the heavy metals employed as catalysts for the air oxidation of aliphatic-substituted benzenes to produce trimellitic acid. A Delta E color of below 2 is often specified for trimellitic anhydride used in white or transparent resins, and a metal Q content of less than about 50 p.p.m. (parts per million) is desirable to achieve good color and oxidation stability. While the more commonly employed maleic and phthalic acid anhydrides are readily prepared by thermal dehydration of the corresponding acids, and the anhydrides are easily purified by atmospheric pressure sublimation, trimellitic anhydride cannot be processed in this manner. Firstly, the acid requires temperatures in excess of 200.degree. C. for thermal dehydration to take place, and even at these temperatures dehydration is not complete. Secondly, Q trimellitic anhydride is essentially nonvolatile and must be distilled at temperatures above 250.degree. C. under vacuums on the order of 10-60 mm mercury absolute to prevent color degradation. Also, to increase the ordinarily slow rate of dehydration, it has previously been proposed to employ chemical dehydrating agents such as acetic anhydride, sulfuric acid, phosphorus pentoxide, or the like to dehydrate the last traces of trimellitic acid before distilling the anhydride. These chemical dehydrating agents are costly to recover and regenerate, and 0 consequently impose an expensive operating burden on existing processes for the purification of trimellitic anhydride. Furthermore, their use in some cases results in the substitution of one impurity for another.
Dimethyl-2,6-naphthalenedicarboxylate is a monomer that can be used to prepare high performance polymeric materials such as polyesters. One such polyester, poly(ethylene-2,6-naphthalate) (PEN), which is suitably prepared by reacting dimethyl-2,6-naphthalenedicarboxylate with ethylene gycol, has exceptional properties making it useful for a variety of applications. Fibers and films made from PEN polyester have improved strength and improved thermal properties relative to other polyester materials. In addition, films made from PEN demonstrate superior resistence to gas diffusion and especially to the diffusion of carbon dioxide, oxygen and water vapor. Due to its exceptional properties, PEN is highly suitable in applications such as food and beverage containers, Q particularly in so called "hot-filled" food and beverage containers, tire cord and magnetic recording tape.
Although PEN is a high performance polyester, its ultimate properties, performance and physical appearance are dependent to a large extent on the purity, including the color, of the dimethyl-2,6-naphthalenedicarboxylate monomer. Therefore, it is important that dimethyl-2,6-naphthalenedicarboxylate used for the preparation of PEN have as high a purity and as low a color as possible. Superior color is a particularly important factor for Q commercial applications because color is an immediately recognizable measure of purity. Impurities in the dimethyl-2,6-naphthalenedicarboxylate can adversely affect polymerization process to which the dimethyl-2,6-naphthalenedicarboxylate is subjected. Additionally, poor color of dimethyl-2,6-naphthalenedicarboxylate may affect the color of PEN.
Accordingly, an object of the present invention is to provide an improved process for preparing high purity trimellitic anhydride from trimellitic acid having a Delta E color below 1.0 and a metal content below 50 p.p.m.
It is also an object of the present invention to provide an improved process for preparing dimethyl-2,6-naphthalenedicarboxylate having improved color.
It has now been discovered that trimellitic anhydride having Delta E color below 1.0 can readily be obtained by treating crude trimellitic anhydride in the presence of activated boric acid followed by fractionation. It has also been discovered that the purity and, in particular, the color of dimethyl-2,6-naphthalenedicarboxylate can be improved by treating crude dimethyl-2,6-naphthalenedicarboxylate with boric acid activated by organic acids or anhydrides, followed by fractionation. Preferred organic acids and anhydrides for activating boric acid are derived from aliphatic hydrocarbons. Prefered anhydrides have about 2 to 8 carbon atoms while preferred acids have 1 to 8 carbon atoms. Most preferred acids and anhydrides include acetic anhydride, acetic acid, propionic acid, propionic anhydride, formic acid, maleic anhydride and 2-ethyl hexanoic acid.
3. Prior Art
In the prior art German Patent Publication 19 48 374, trimellitic acid is treated with boric acid. In our improved process, trimellitic anhydride is treated with activated boric acid of about 200.degree. to about 275.degree. C. and treated with a decreased pressure and distilled under further decreased pressure at a temperature of about 200 .degree. to about 275.degree. C.